Influence of furnace baking on Q-E behavior of superconducting accelerating cavities
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Prog. Theor. Exp. Phys. 2021, 071G01 (8 pages)
DOI: 10.1093/ptep/ptab056
Letter
Influence of furnace baking on Q–E behavior of
superconducting accelerating cavities
H. Ito1,∗ , H. Araki1 , K. Takahashi2 , and K. Umemori1,2
1
High Energy Accelerator Research Organization (KEK), 305-0801 Tsukuba, Ibaraki, Japan
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2
The Graduate University for Advanced Studies, SOKENDAI, 305-0801 Tsukuba, Ibaraki, Japan
∗
E-mail: hayato.ito@kek.jp
Received February 26, 2021; Revised April 6, 2021; Accepted April 27, 2021; Published May 6, 2021
...................................................................................................................
The performance of superconducting radio-frequency (SRF) cavities depends on the condition
of the niobium surface. Recently, various heat treatment methods have been investigated to
achieve an unprecedentedly high quality factor (Q) and high accelerating field (E). We report the
influence of a new baking process called furnace baking on the Q–E behavior of 1.3 GHz SRF
cavities. Furnace baking is performed as the final step of the cavity surface treatment; the cavities
are heated in a vacuum furnace for 3 h, followed by high-pressure rinsing and radio-frequency
measurement. This method is simpler and potentially more reliable than previously reported heat
treatment methods, and it is therefore easier to apply to the SRF cavities. We find that the quality
factor is increased after furnace baking at temperatures ranging from 300 ◦ C to 400 ◦ C, while a
strong decrease in the quality factor for a high accelerating field is observed after furnace baking
at temperatures ranging from 600 ◦ C to 800 ◦ C. We find significant differences in the surface
resistance for various processing temperatures.
...................................................................................................................
Subject Index G05
1. Introduction A superconducting radio-frequency (SRF) cavity is a key component of par-
ticle accelerators used to generate charged-particle beams. An SRF cavity exhibits lower energy
dissipation and lower surface resistance (Rs ) under a radio frequency (RF) field compared to a
normal-conducting accelerating cavity, which enables continuous-wave operation with a high accel-
erating field (Eacc ). Owing to decades of research focused on the improvement of SRF cavities
[1,2], various surface treatment techniques have been established; thus, SRF cavities with superior
performance in terms of the quality factor (Q0 ) and Eacc have been developed [3–5].
In the standard surface treatment planned for the International Linear Collider (ILC), the follow-
ing procedure is implemented: after fabricating the SRF cavity, a 100 μm layer of the cavity inner
surface is removed by bulk electropolishing; this results in the elimination of the surface layer dam-
aged during cavity fabrication. After electropolishing, the surface is thoroughly rinsed with ultrapure
water, and ultrasonic cleaning is performed by filling ultrapure water with a surfactant, followed by
high-pressure ultrapure water rinsing (HPR). Then, annealing is performed in a vacuum furnace at
approximately 800 ◦ C to desorb the hydrogen that was absorbed on the niobium surface during elec-
tropolishing. Subsequently, light electropolishing is performed to remove a layer of approximately
20 μm of the cavity inner surface to eliminate dirt from the inner surface, followed by sufficient water
rinsing, ultrasonic cleaning, HPR, and assembly in a cleanroom. Next, as the final step in the surface
treatment process, the cavity is vacuumed and heat-treated at 120 ◦ C for 48 h. This heat treatment
© The Author(s) 2021. Published by Oxford University Press on behalf of the Physical Society of Japan.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/),
which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.PTEP 2021, 071G01 H. Ito et al.
suppresses a phenomenon called high field Q slope (HFQS), in which Q0 decreases significantly at
high Eacc [6], and further improves Eacc .
In recent years, further surface treatment techniques, such as nitrogen doping [7,8], nitrogen
infusion [8,9], and two-step baking [10], have been investigated to increase Q0 and Eacc . The nitrogen-
doped cavities have an extremely high Q0 and show increasing Q0 as a function of Eacc , which is
referred to as the anti-Q slope. However, the maximum Eacc obtained with nitrogen doping is lower
than for the standard surface-treated cavity. Moreover, nitrogen-doped cavities are highly sensitive
to trapped magnetic flux compared with standard treated cavities [11]. The nitrogen doping process
has been applied in the Linac Coherent Light Source (LCLS-II) cavity fabrication process [12,13]
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because it is highly reproducible and has resulted in high Q0 and anti-Q slope in several studies
[8]. In the nitrogen infusion technique, the Q–E behavior does not change significantly, and both
Q0 and Eacc are improved compared with the standard surface treatment methods [14–16]; how-
ever, reproducibility has been limited to a few laboratories. The two-step baking process developed
at Fermi National Accelerator Laboratory (FNAL) can produce cavities with a maximum Eacc of
approximately 50 MV m−1 [10,17], and other laboratories are currently verifying the effectiveness
of two-step baking.
In this study, a new heat treatment method, which is simpler and more reliable than the surface
treatment method described above, is investigated from the viewpoint of oxygen diffusion of a
niobium oxide layer, and the effects on the properties of Q0 , the Bardeen–Cooper–Schrieffer (BCS)
resistance (RBCS ), and the residual resistance (Rres ) for each Eacc are studied. In the 1980s and
1990s, SRF cavities that were heat treated in vacuum in the ranges of 250 ◦ C to 300 ◦ C and 1100 ◦ C
to 1400 ◦ C were investigated to understand the effect of an oxide layer on the SRF cavity performance
[18–20]. It was revealed that the heat treatment at 250 ◦ C dissolves the oxide layer and decreases
RBCS . Therefore, it is expected that the heat treatment in this study can be performed in the same
temperature range to create a cavity with high Q0 .
2. Experiment We used several 1.3 GHz TESLA and STF (TESLA-like) single-cell cavities that
had undergone various surface treatments. As a first step, a 10 or 20 μm layer of the cavity inner
surface was removed by light electropolishing to reset the surface conditions in the cavity, followed
by HPR to eliminate any remaining impurities on the surface. Subsequently, the cavities were placed
in a large vacuum furnace. The inner diameter of the furnace chamber is 950 mm, and the length
is 2080 mm [21]. This vacuum furnace can be depressurized to 1 × 10−6 Pa at room temperature
using a cryopump. Due to the insufficient cooling capacity of the cryopump, heat treatment at 800 ◦ C
increases the temperature of the cryopump. In such a case, we switched from the cryopump to a turbo
molecular pump to achieve the desired level of vacuum [21]. The vacuum furnace was equipped with
a quadrupole mass spectrometer (Q-mass) to monitor the partial pressure of each element during
heat treatment.
The cavities were baked in a temperature range of 200 ◦ C to 800 ◦ C for 3 h in this vacuum
furnace. This baking process is referred to as “furnace baking,” which is different from the “medium-
temperature bake” (mid-T bake) that is performed at FNAL and the Institute of High Energy Physics
(IHEP) [22,23]. The mid-T bake process requires special heat treatment equipment to perform the
RF measurement without exposing the inner surface of the cavity to the air after heat treatment at
250 ◦ C to 400 ◦ C, whereas the furnace baking process is a simple method that can be performed with
existing cavity treatment systems because the heat treatment is performed in a vacuum furnace. Note
that the light electropolishing is always applied at the beginning of each furnace baking process to
2/8PTEP 2021, 071G01 H. Ito et al.
104
X-ray [μSv/h]
Q0
4×1011
3×1011
2×1011
103
1011
4×10
10 102
3×1010
10
2×10
1.5 K 10
1010 1.6 K
1.7 K
9
1.8 K
4×10 1.9 K
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9 1
3×10
2.0 K
9
2×10
Radiation at 2 K
10−1
0 5 10 15 20 25
Eacc [MV/m]
Fig. 1. Q–E curve at each temperature for a cavity that was furnace baked at 350 ◦ C. The colored circles show
the Q–E curves at each temperature, and the purple squares show the radiation levels at the 2 K measurement.
reset the surface conditions in the cavity. To perform the furnace baking, the cavity temperature was
ramped up from room temperature at a ramp rate of 200◦ C h−1 to a target temperature of 200 ◦ C to
800 ◦ C and then the target temperature was held for 3 h. The vacuum was 1–2 × 10−6 Pa at room
temperature, and it was maintained at the order of 10−4 Pa even during baking. After the furnace
baking process and cooling down to below 50 ◦ C, the furnace was purged with N2 gas, and the cavity
was packed and placed on the HPR stand for final rinsing before assembly. A new niobium oxide
layer grows on the inner surface during this step because of exposure to air and water; however, this
is not expected to affect the oxygen diffusion region formed by furnace baking. After assembly, no
further baking was performed, and the RF measurements were conducted.
The cavity was then cooled down by depressurizing liquid helium to 1.5 K, which is the lowest
temperature that can be achieved in the cryostat available at the High Energy Accelerator Research
Organization (KEK). Then, the Q–E curve at each temperature was obtained by calculating the input,
reflected, and transmitted RF powers for 0.1 K temperature increments. Finally, the RF measurement
was performed up to the quench field at 2 K. To minimize the magnetic flux trapping during the cooling
process, the magnetic field around the cavity was reduced to less than ∼1 mG using the magnetic
shield and solenoid coil, and a heater placed at the top beam tube was used to provide a temperature
gradient in the cavity for flux expulsion [24–26].
3. Results Figure 1 shows the Q–E curve for a cavity that was furnace baked at 350 ◦ C. The
cavity was quenched once during the measurement at 1.5 K, which caused it to trap magnetic flux
and subsequently decrease Q0 . However, a higher Q0 and anti-Q slope were still clearly observed in
the 2 K measurement results compared to the standard treated cavity (see Fig. 3).
Rs at each temperature and Eacc is calculated using Rs = G/Q0 , where G is the geometric factor that
is independent of material properties [27]. Rs can be expressed as the sum of RBCS , which decreases
exponentially with temperature, and Rres , which is a weak temperature-dependent or temperature-
independent term that cannot be accounted for in RBCS . Rs is decomposed into RBCS and Rres at each
Eacc using the dataset at the same Eacc from the Q–E curve. The decomposition is performed using
3/8PTEP 2021, 071G01 H. Ito et al.
2×10−8 10
Rs [Ω]
Rres [nΩ]
9 Rres
Rres Before trap
Rs = p0 1 exp(-p1 1) + p2
T T 8 RBCS at 2.0 K
and
−8 at 2 MV/m
10 7
RBCS
9×10−9 at 4 MV/m
8×10−9 at 6 MV/m 6
7×10−9 at 8 MV/m
5
6×10−9 at 10 MV/m
−9
5×10 at 12 MV/m 4
at 14 MV/m
−9
4×10 at 16 MV/m
3
2
3×10−9
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1
−9
2×10 0
0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0 5 10 15 20 25
1/T [/K] Eacc [MV/m]
Fig. 2. Temperature dependence of Rs for each Eacc (left) and behavior of RBCS and Rres for Eacc (right). Rres
before the trap was obtained by subtracting RBCS obtained after the trap from Rs at 1.5 K before the trap.
the following fitting equation:
Aω2 −(/kT )
Rs (T ) = e + Rres , (1)
T
where A is a fitting constant that depends on the superconducting properties, T is the temperature, k
is the Boltzmann constant, 2 is the energy gap of the superconductor, which is treated as a fitting
parameter, and ω is the frequency of the cavity. The first term in this equation corresponds to RBCS .
The colored curves in the left panel of Fig. 2 show the fitting parameters for Rs (T ) at each Eacc . The
red closed circles in the right panel of Fig. 2 illustrate the behavior of Rres for Eacc . The blue closed
circles depict the behavior of RBCS , which decreases sharply as Eacc increases in the case of 350 ◦ C
furnace baking. This behavior is considerably different from the behavior of standard treated cavities
(120 ◦ C and 48 h baked cavities), and it is similar to the behavior observed in a nitrogen-doped cavity
[7]. The red open circles depict the estimated behavior of Rres before the flux trapping; Rres is smaller
than for standard treated cavities for 350 ◦ C furnace baking.
Figure 3 shows a comparison of the Q–E curves measured at 2 K for cavities that were furnace baked
at various temperatures (200 ◦ C to 800 ◦ C) and a standard treated cavity (120◦ C and 48 h baking
under vacuum directly followed by RF measurement, no exposure to air or water). The Q–E behavior
of the 200 ◦ C and 3 h furnace-baked cavity (purple points) is similar to that of the standard-treated
cavity (black points). This indicates that the conventional performance can be achieved by replacing
the 120 ◦ C and 48 h baking with 200 ◦ C and 3 h furnace baking, which may be highly effective
for mass production of cavities. Four furnace-baked cavities, baked at 300 ◦ C to 400 ◦ C, have high
Q0 and anti-Q slope, but a low Eacc compared with the standard treated cavity. This behavior is
typically associated with nitrogen-doped cavities [7]. In particular, 300 ◦ C furnace baking produces
an extremely high Q cavity, with a Q0 of over 5 × 1010 at 16 MV m−1 . Furthermore, 300◦ C furnace
baking has the same effect for two different cavities, indicating good reproducibility. These results
are in good agreement with those obtained for single-cell cavities that are furnace-baked in the
temperature range of 250 ◦ C to 400 ◦ C at IHEP [28].
The high-temperature furnace-baked cavities baked at 600 ◦ C and 800 ◦ C did not reach high Q0 ,
and the Q values were comparable to those of the standard-treated cavity, while HFQS was observed
in these cavities. This HFQS is considered to be related to the diffusion of oxygen and hydrogen
4/8PTEP 2021, 071G01 H. Ito et al.
1011 104
X-ray [μSv/h]
Q0
5×1010
4×1010 103
10
3×10
2×1010
102
120C 48 h baking (R-9 15th VT)
200C 3 h baking (R-4 11th VT)
1010 300C 3 h baking (R-4 10th VT)
300C 3 h baking (R-8 13th VT)
350C 3 h baking (R-4 13th VT)
400C 3 h baking (TE1AES018 7th VT) 10
9 600C 3 h baking (R-4 12th VT)
5×10 800C 3 h baking (R-8c 5th VT)
9
4×10 Radiation of 300C 3 h baking (R-4)
9
3×10 Radiation of 350C 3 h baking
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Radiation of 400C 3 h baking 1
2×109
10−1
0 5 10 15 20 25 30 35 40
Eacc [MV/m]
Fig. 3. Comparison of Q–E behavior measured at 2 K for cavities that were furnace baked at various temper-
atures (200 ◦ C to 800 ◦ C) and a standard-treated cavity (120 ◦ C and 48 h baking). The colored closed points
show the Q–E curve for each furnace-baked cavity, and the colored open points show the radiation levels
corresponding to each colored closed point.
on the inner surface of the cavity [29–33], and the effect of suppressing the HFQS diminished due
to oxygen diffusion at high temperatures. These results indicate that diverse Q–E behaviors were
obtained when the furnace baking temperature was changed from 200 ◦ C to 800 ◦ C. In particular,
furnace baking at 300 ◦ C to 400 ◦ C resulted in high Q0 and anti-Q slope. The low Eacc is similar to
that of the nitrogen-doped cavity and may be related to the low superheating field at the dirty limit
[34]. Theoretical considerations suggest that Q0 varies depending on the cavity surface condition
[35–37].
4. Discussions The left panel of Fig. 4 shows the relationship between RBCS at 2 K and Eacc
for each furnace-baked and standard-treated cavity. RBCS behavior is classified into three types: one
that increases with increasing Eacc , one that decreases with increasing Eacc , and one that does not
increase as much as the first type. The 200 ◦ C furnace-baked cavity and the standard-treated cavity
correspond to the first type mentioned above, and the slope of RBCS is steep compared with those
obtained for other cavities. The 300 ◦ C to 400 ◦ C furnace-baked cavities correspond to the second
type. In these cavities, RBCS decreases as Eacc increases, which is the origin of the anti-Q slope.
This behavior is the most pronounced in 300 ◦ C furnace-baked cavities. The 600 ◦ C and 800 ◦ C
furnace-baked cavities correspond to the third type. These cavities already have a high RBCS at low
Eacc ; however, the slope is less steep compared with the first type. From these results, it was found
that RBCS behavior varies significantly with differences in the baking temperature, resulting in the
variation of Q–E behavior. Further, it was suggested that there is an inflection point between 200 ◦ C
and 300 ◦ C, where the behavior of RBCS changes significantly, and that a similar inflection point
exists in the region between 400 ◦ C and 600 ◦ C. The right panel of Fig. 4 shows the relationship
between Rres and Eacc for each furnace-baked and standard-treated cavity. It was found that Rres is
lower for all the furnace-baked cavities compared with the standard-treated cavity, and Rres behavior
changes with differences in baking temperature but not as drastically as the RBCS behavior. Notably,
the 600 ◦ C furnace-baked cavity has an extremely low Rres of 0.2 n, which corresponds to a Q0
of over 1 × 1012 . Because Rres dominates Rs at temperatures of approximately 1 K, this 600 ◦ C
5/8PTEP 2021, 071G01 H. Ito et al.
10
RBCS [nΩ]
Rres [nΩ]
14 120C 48 h baking (R-9 15th VT)
9 200C 3 h baking (R-4 11th VT)
300C 3 h baking (R-4 10th VT)
300C 3 h baking (R-8 13th VT)
12 8
350C 3 h baking (R-4 13th VT)
400C 3 h baking (TE1AES018 7th VT)
7 600C 3 h baking (R-4 12th VT)
10 800C 3 h baking (R-8c 5th VT)
6
8
5
6 4
120C 48 h baking (R-9 15th VT) 3
4 200C 3 h baking (R-4 11th VT)
300C 3 h baking (R-4 10th VT)
300C 3 h baking (R-8 13th VT) 2
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350C 3 h baking (R-4 13th VT)
2 400C 3 h baking (TE1AES018 7th VT)
600C 3 h baking (R-4 12th VT)
1
800C 3 h baking (R-8c 5th VT)
0 0
0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40
Eacc [MV/m] Eacc [MV/m]
Fig. 4. Left: RBCS behavior at 2 K for Eacc for each furnace-baked cavity and standard-treated cavity. Right:
The relationship between Rres and Eacc for each furnace-baked cavity and standard-treated cavity.
furnace baking has the potential to be a useful processing method for superconducting devices, such
as those used at cryogenic temperatures in the mK region rather than the SRF accelerator application
operating at 2 K.
The sensitivity of the mid-T (300 ◦ C to 400 ◦ C) furnace-baked cavity was estimated by comparing
the Q–E curve after cooling with the flux expulsion inPTEP 2021, 071G01 H. Ito et al.
3
Sensitivity [nΩ/mG]
120C 48 h baking (standard recipe)
300C 3 h baking (furnace baking)
2.5 350C 3 h baking (furnace baking)
400C 3 h baking (furnace baking)
2
1.5
1
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0.5
0
0 5 10 15 20 25 30 35 40
Eacc [MV/m]
Fig. 5. Sensitivity of mid-T furnace-baked and standard-treated cavities.
low, its high Q0 is very impressive, and combined with the simplicity of the furnace baking proce-
dure, it is clear that mid-T furnace baking can be successfully adapted to various SRF applications
in the future. Rres is lower for all the furnace-baked cavities compared with standard-treated cavities.
For the 600 ◦ C furnace-baked cavity, an extremely low Rres of 0.2 n is obtained. The sensitivity
of the mid-T furnace-baked cavity is higher than the standard-treated cavity and resembles that of
the nitrogen-doped cavity. The 300 ◦ C furnace-baked cavity, which has the highest Q0 , has a higher
sensitivity than the nitrogen-doped cavity. Further studies will be undertaken by focusing on surface
analysis based on the sample study to reveal the relationship between these behaviors and the cavity
surface condition. Further, process optimization will be performed by changing the baking time.
Acknowledgements
This work was supported by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific
Research(B) No. 19H04402.
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